1. Field of the Invention
The present invention relates to a two-dimensional image read-out device, and more particularly to an image read-out device wherein a photoconducting layer is formed in a two-dimensional plane as wide as the read-out region of the device.
2. Description of the Prior Art
A two-dimensional image read-out device converts a two-dimensional pattern of light into an electric signal, and has been used in an image processing appliance to read out images such as characters and drawings on a manuscript. A compact two-dimensional image read-out device is available which makes use of the photoconductivity of a semiconductor film such as CdSe and amorphous Si.
FIG. 2 is a sectional view of this type of prior art two-dimensional image read-out device. On an electrically insulating substrate 1, X stripe electrodes 2, a photoconducting layer 3, and Y stripe electrodes 4 are applied successively. The X stripe electrodes 2 are arranged parallel to each other with a constant pitch. The Y stripe electrodes 4 are arranged similarly parallel to each other with a constant pitch, but in a direction perpendicular to the X electrodes 2. Each intersection between the X and Y electrodes forms a picture element. An X-Y matrix of picture elements detects a two-dimensional pattern of light from an image to be read out. If the Y electrodes 4 are made transparent or translucent, picture elements are exposed to the light from above through the Y electrodes 4.
This type of a two-dimensional image read-out device can convert the intensity of the incident light into an electric signal as follows. A two-dimensional image read-out device which can be made as wide as a manuscript is set just above the manuscript, with a lens interposed therebetween. The light from an image on the manuscript enters the picture elements, and the resistance between the X and Y electrodes through the photoconducting material in each picture element changes with the intensity of the incident light on the picture element. The change in resistance is by a circuit shown in FIG. 1. An external voltage is applied between either the X or Y electrode and a load resistance R.sub.L via the photoconducting layer and the other of the Y or X electrode. Then, an output signal which is proportional to the current flowing across the load resistance varies as the incident light on a picture element becomes light or dark. That is, an image is distinguished as being light or dark by each picture element in a two-dimensional array of picture elements.
Since the prior art two-dimensional image read-out device has such a structure as shown in FIG. 2, the current flows in two kinds of paths through the photoconducting layer 3 between the X electrode 2 and the Y electrode 4. When the light is incident on the picture element, the current flows along the intended path, that is, through the picture element. On the other hand, if the light is incident near the picture element at the same time, the current flows also in a circuitous path, that is, through a photoconducting material surrounding the picture element, the electric resistance of which is lowered by the incident light. In other words, the output signal is not only ascribable to the intensity of the light incident on the picture element, but the effect of the leakage current on the output signal should also be taken into account. Thus, the dynamic range of the conversion from a light signal to an electrical signal cannot be set too large because it necessarily enhances the instability of the output signal. If the intensity of the light surrounding the picture element becomes large while the intensity of the light at the picture element itself is low, a malfunction will occur. Namely, an output signal which should correspond to the dark state is transformed to a signal corresponding to the light state. If the Y electrodes are made of a material with low light transmissinity, the intensity of the light entering a picture element through the Y electrode to the photoconducting layer diminishes and the above-described trouble is enhanced.
An object of the present invention is to overcome the abovementioned troubles with a two-dimensional image read-out device, or to provide a two-dimensional image read-out device wherein the passage of light is obstructed at the surroundings of picture elements so that the dynamic range can be set large.